Information display illumination for direct or backlighting purposes is well known to those skilled in the art. Conventional information display devices such as a liquid crystal display require the presence of a light source in order to provide contrast which makes the display readable. When in high ambient lighting conditions such as sunlight, the liquid crystal display requires no additional illumination source in order to provide sufficient contrast.
However, in low ambient lighting conditions, display illumination is accomplished in conventional information display devices by the use of incandescent bulbs or electro-luminescent display panels. Incandescent bulbs, when used as a means for display backlighting, yield significant undesirable bright spots in the areas of the display near their physical locations. Many attempts have been made to uniformly diffuse the light emitted from incandescent sources to provide a more consistent illumination of the information display area. The present trend in electronic information display devices is to use electro-luminescent display panels as backlighting devices. When used as a backlighting illumination device, the electro-luminescent display panel solves the problems of bright spots associated with the use of incandescent bulbs as backlighting devices. With electro-luminescent display panels, designers now have an excellent means of providing a uniform illumination for backlighting a liquid crystal display when viewed in low ambient lighting conditions.
However, electro-luminescent display panels are not without their problems. The most significant problem facing the design engineer after selecting the electro-luminescent display panel for illumination of an information display is the high drive voltage required to effectively illuminate an electro-luminescent display panel. The higher (or medium) voltage requirements of EL panel results in extra circuitry to generate the higher voltage from the conventional low voltage supply of selective call receivers. Typically, the present art generates another high voltage supply by using a second booster converter, which may be generated by additional circuitry.
FIG. 1 shows a block diagram of a conventional drive circuit for a electroluminescent (EL) panel 110. According to this configuration, an input clock within the timing logic 112 provides the necessary timing for the operation of the EL panel drive 114. During one phase of the panel drive 114, the EL1 side of the EL panel 110 is coupled to ground, and the EL2 side is coupled to the coil 108 through the internal diode 102. By rapidly opening and closing the coil switch 118 within the coil drive 116, the EL panel 110 is charged to its operating voltage (typically seventy volts). When the EL panel 110 reaches its operating voltage the phase of the EL lines (EL1, EL2) are reversed, thus connecting the EL2 side of the EL panel 110 to ground, and connecting the EL1 side to the coil 108 through the internal diode 102. Similar switching operation of the coil switch 118 charges the EL panel 110 to its operating voltage as described above.
As further shown, the EL panel 110 and the timing logic 112 are conventionally supplied from a single power supply (B+) from which the high voltage for powering the EL panels is generated. However, the power supply (B+) would not provide an economical solution for many contemporary small devices, such as, selective call receivers, because a high voltage supply would severely increase the cost of the device.
Alternative methods for generating a high voltage from a low voltage battery includes the known techniques of voltage multiplication. However, the power supply of small devices (e.g., selective call receiver) is about one and half volts, which is insufficient to generate the timing logic 112. Therefore, a large and more expensive power supply (B+) capable of providing a higher voltage and current is used, thus failing to provide a cost effective solution for various voltage supplies. Accordingly, a need exists for a cost-effective method of generating secondary voltages power supplies to permit the use of lower voltage.